client: log error when syncCancel op dtor on worker

Log an error when an operation created with syncCancel=true
is allowed to be free'd asynchronously on the client worker.

This may indicate that the Operation/Subscription is directly
owned by its own callback.  Creating a self-reference loop.
Or it might indicate a more complicated, but safe, situation
like pvalink where a new operation is initiated as the previous
one completes.

Use the syncCancel=false flag, opting of blocking cancellation,
as an indication that this risk is accepted by user code.

Author: mdavidsaver

ioc/pvalink_channel.cpp

@@ -267,6 +267,7 @@ void pvaLinkChannel::put(bool force)
         // start net Put, cancels in-progress put
         op_put = linkGlobal->provider_remote.put(key.first)
                 .rawRequest(pvReq)
+                .syncCancel(false)
                 .build([this](Value&& prototype) -> Value
         {
                 return linkBuildPut(this, std::move(prototype)); // TODO

src/clientdiscover.cpp

@@ -93,6 +93,12 @@ std::shared_ptr<Operation> DiscoverBuilder::exec()
         // (maybe) user thread
         auto loop(op->context->tcp_loop);
         auto temp(std::move(op));
+        if(syncCancel && loop.inLoop())
+            log_err_printf(io,
+                           "syncCancel discover '%s' being destroyed on worker thread.\n"
+                           "Possible self-reference loop.  Setup with .syncCancel(false) if intended.\n",
+                           temp->channelName.c_str());
+
         loop.tryInvoke(syncCancel, std::bind([](std::shared_ptr<Discovery>& op){
                            // on worker
                            op->context->discoverers.erase(op.get());

src/clientget.cpp

@@ -613,6 +613,12 @@ std::shared_ptr<Operation> gpr_setup(const std::shared_ptr<ContextImpl>& context
         // (maybe) user thread
         auto temp(std::move(internal));
         auto loop(temp->loop);
+        if(syncCancel && loop.inLoop())
+            log_err_printf(io,
+                           "syncCancel op%u '%s' being destroyed on worker thread.\n"
+                           "Possible self-reference loop.  Setup with .syncCancel(false) if intended.\n",
+                           temp->op, temp->channelName.c_str());
+
         // std::bind for lack of c++14 generalized capture
         // to move internal ref to worker for dtor
         loop.tryInvoke(syncCancel, std::bind([](std::shared_ptr<GPROp>& op) {

src/clientintrospect.cpp

@@ -207,6 +207,12 @@ std::shared_ptr<Operation> GetBuilder::_exec_info()
         // from user thread
         auto temp(std::move(op));
         auto loop(temp->loop);
+        if(syncCancel && loop.inLoop())
+            log_err_printf(io,
+                           "syncCancel info '%s' being destroyed on worker thread.\n"
+                           "Possible self-reference loop.  Setup with .syncCancel(false) if intended.\n",
+                           temp->channelName.c_str());
+
         // std::bind for lack of c++14 generalized capture
         // to move internal ref to worker for dtor
         loop.tryInvoke(syncCancel, std::bind([](std::shared_ptr<InfoOp>& op) {

src/clientmon.cpp

@@ -830,6 +830,12 @@ std::shared_ptr<Subscription> MonitorBuilder::exec()
         // from user thread
         auto temp(std::move(op));
         auto loop(temp->loop);
+        if(syncCancel && loop.inLoop())
+            log_err_printf(io,
+                           "syncCancel monitor '%s' being destroyed on worker thread.\n"
+                           "Possible self-reference loop.  Setup with .syncCancel(false) if intended.\n",
+                           temp->channelName.c_str());
+
         // std::bind for lack of c++14 generalized capture
         // to move internal ref to worker for dtor
         loop.tryInvoke(syncCancel, std::bind([](std::shared_ptr<SubscriptionImpl>& op) {

src/evhelper.cpp

@@ -348,6 +348,11 @@ bool evbase::_call(mfunction&& fn, bool dothrow) const
     return true;
 }
 
+bool evbase::inLoop() const
+{
+    return pvt->worker.isCurrentThread();
+}
+
 void evbase::assertInLoop() const
 {
     if(!pvt->worker.isCurrentThread()) {

src/evhelper.h

@@ -198,6 +198,7 @@ struct PVXS_API evbase {
             return tryDispatch(std::move(fn));
     }
 
+    bool inLoop() const;
     void assertInLoop() const;
     //! Caller must be on the worker, or the worker must be stopped.
     //! @returns true if working is running.